A useful reminder from Nicholas Christakis in his superb new book about the impact of coronavirus on the way we live:
Masks alone can therefore have a large effect on respiratory pandemics by bringing the Re down. For instance, a mask with just 50 percent efficacy in reducing droplet transmission worn by just 50 percent of people can reduce the Re from 2.4 to about 1.35—roughly the level of seasonal influenza. This means that, if there were one hundred cases of such an infection at the beginning of the month, in a no-mask scenario, there would be 31,280 cases at the end of the month; but in a mask scenario, there would be only 584. This reduction allows medical personnel to take better care of the smaller number of patients and to deploy contact tracing and quarantine measures more effectively. Of course, if the masks were even more efficient and the adherence even higher, the epidemic could be brought to heel, with an Re lower than 1.0. If 70 percent or more of the population in a typical urban situation used masks of decent quality (that is, about 70 percent effective), it would prevent a large-scale outbreak of a respiratory pathogen of a moderately contagious disease such as COVID-19.35
Apollo’s Arrow: The Profound and Enduring Impact of Coronavirus on the Way We Live, Pg 105
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